Method and apparatus for coating paper



1955 J. P. BREZINSKI 3,202,536

METHOD AND APPARATUS FOR COATING PAPER Filed Nov. 8, 1961 2 Sheets-Sheet1 IN VEN TOR. (/9/0/929 1. Erezinmi g- 1965 J. P. BREZINSKI METHOD ANDAPPARATUS FOR COATING PAPER 2 Sheets-Sheet 2 Filed Nov. 8, 1961 IN V ENTOR Jew/we I? Ema/ms?! United States Patent 3,2il2,536 METHOB ANDAPPARATUS F R COATING PAPER Jerome P. Brezinski, Beloit, Wis, assignorto Beloit Corporation, Beloit, Wis, a corporation of Wisconsin FiiedNov. 8, 1961, Ser. No. 1513981 19 Claims. (Cl. 11783) This is acontinuation-in-part of my application Serial No. 806,542, filed April15, 1959, and now abandoned.

This invention relates to improvements in coating porous webs such aswebs of paper and the like and more particularly relates to a method andapparatus for doublewet coating a web of paper wherein a blade coatermay be used to smooth a first relatively high viscosity coating layer towhich is directly applied while such layer is wet, and at the samestation, a relatively low viscosity coating layer which may then besmoothed by an air knife.

The so'called air-brush or air-knife method of coating is disclosed, forexample, in United States Patent No. 1,980,923 to Label and in improveddetail in United States Patent No. 2,139,628 to Terry. It is known thatsuch an air-knife coating apparatus which normally comprises means suchas a roller or backup roll for supporting one side of the web and meansto apply a substantially nondivergent jet of air emitted from a straightslit orifice of a nozzle to the coating on the web tends to leave auniformly thick layer of coating on the web. In practice, the jet of airissues from the nozzle with substantially uniform velocity throughoutits entire extent and impinges upon the wet coating at an angle and in adirection opposite to the direction of web travel, thereby cutting awaythe excess coating and leaving the desired quantity of coating on theweb in a layer of uniform thickness. This characteristic has been usedto produce semi-coated grades of paper in accordance with the teachingsof, for example, US. Patent No. 2,325,798 to L. W'. Porter. As indicatedin Porter, the type of fluid coating composition suitable for usetherein and in the instant invention is one in which a solid componentis suspended in a liquid carrier or vehicle. The instant invention willbe described in connetcion with coating a paper body-stock with amineral-coating composition that is a fluid composition comprising apigment component (e.g., clay, calcium carbonate, ochre, aluminumpowder, satin white, titanium dioxide and the like) and an adhesivecomponent (cg, starch, casein, vegetable protein, or equivalent)suspended in an aqueous vehicle.

The finished quality of any coated paper prepared with an air-knifecoater, however, depends to a considerable extent on the quality of thepaper sheet or web. In particular, the air jet will remove coating fromthe cavities and pores of the base sheet surface with the result thatsuch defects are also found in the surface of the coated sheet. The verycharacteristic which gives the air jet technique one of its advantagesleads also to this disadvantage. Inasmuch as the air jet produces, asnoted above, a uniformly thick layer of coating, it follows that anyirregularities in the base sheet or web will also be reproduced in thesurface of the coated paper where a single coating layer is smoothed byair jet techniques. On the other hand, multiple layer coating techniquesrequiring a drying step between the application of successive layers inorder to achieve a smooth surfaced paper unduly increase the cost of thefinal product.

Another problem of particular significance in the coating art resides inthe fact that many coating compositions that may be easily applied to apaper web develop weaknesses or imperfections therein upon drying suchthat these coatings tend to fail in their adhesion to the web andseparate therefrom. This has been found to be par- 3,22,53h PatentedAug. 24, 1965 ticularly true in the case of relatively low viscosity(i.e., low solids content) coatings.

It is, therefore, an important object of the instant invention toprovide an improved method and apparatus for coating paper whichovercomes the above noted difliculties in the art.

It is a further object of the instant invention to provide a method andapparatus for coating paper by which two successive layers of coatingmay be applied at a single coating station without an intermediatedrying step.

It is a further object of this invention to provide a method andapparatus of coating paper wherein an airlmife may be used to remove theexcess of the final layer of coating without affecting the earlier layeror layers and without reproducing web irregularities in the finalsurface.

Other objects, features and advantages of the present invention willbecome apparent to those skilled in the art from the following detaileddisclosure thereof and the drawings attached hereto and made a parthereof.

On the drawings:

FIGURE 1 is a diagrammatic illustration of a first embodiment ofapparatus suitable for carrying out the method of the present invention;

FIGURE 2 is a diagrammatic illustration of a second embodiment ofapparatus suitable for carrying out the method of the present invention;

FIGURE 3 is an essentially diagrammatic fragmentary elevational viewshowing the paper web of the invention at various stages of the coatingprocess; and

FIGURE 4 is still another diagrammatic illustration of .a thirdembodiment of apparatus suitable for carrying out the method of thepresent invention.

As shown on the drawings:

Although it i not desired to limit the invention to any particulartheory, reference is made initially to the diagrammatic showing ofFIGURE 3 to explain what are believed to be the theoreticalconsiderations involved in distinguishing the instant coating method andpaper coated thereby from the prior art. In FIGURE 3, on the left handside the reference numeral 19 indicates generally the paper web to becoated at an initial stage in the coating process and the referencenumeral 10a on the right hand side of FIGURE 3 indicates the same paperweb in a subsequent coating step. The initial coating indicatedgenerally by the reference numeral 11 is here represented by a pluralityof small circles representing diagrammatically the solid particles thatare suspended in the aqueous solution of adhesive such as starch,casein, and the like. The initial coating 11 may be applied to thesurface of the paper 10 by means of a blade coater shown fragmentarilyat 12, which generally serves to fill the valleys or surfaceirregularities indicated diagrammatically at 13 and 1 5 in the Web 10with the coating 11 and present a relatively smooth surface 15 for theouter surface of the coating .11. The blade 12 may be the functioningpart of a typical trailing blade coater, which is capable of applyingcoatings throughout a substantial range of viscosities, including therather high viscosity coatings. Even with relatively viscous coatingcompositions 11, however, it has been found that the trailing blade 12or other mechanical smoothing means may result in the formation of arelatively smooth surface 15, while still substantially eliminating theeffect of the rather great surface irregularities 13, 14 in the web 10.This i the way in which the instant invention is carried out, using acoating composition 11 of relatively high viscosity to form the initiallayer on the Web 16 with a comparatively smooth surface 15, although notnecessarily a surface having the characteristics desired for top qualitypublication grade papers or the like.

if, in contrast, a coating composition 11 of relatively low viscosity isapplied to the initial uncoated web 10, in accordance with the practiceof the prior art, it has been found that when the coating 11 ultimatelydries, there is a tendency for the coating to fail along a planeindicated generally at 16 that is very closely adjacent to theapproximate plane of the interface (indicated generally at 17) betweenthe web and the coating "11.

rather substantial migration of the aqueous phase of the coatingcomposition 11. In order words, it is believed that as soon as thecoating 11 is applied to the web 10 capillary action (or relatedphenomenaltends. to draw :a substantial portion of the aqueous system orvehicle carrying the solid particles in the coating ll'into the body ofthe Web 10 in an area designated in dot-ted *lines generally at 18. Theaqueous system drawn into layer 11, but the water thus migrating back tobe evaporated by suitable drying means does not carry back into thecoating layer any appreciable portion of the very substantial quantityof adhesive which it initially conveyed into the web area 18. This it isbelieved results in a shortage-of adhesive in a relatively localized'area,'-i.e., along the plane 16 in the coating 11, o that the coatingtends to fail.

It has been found 'tha the phenomenon of the weakness of the driedcoating layer along the plane 16 is reduced, if either the viscosity orth solids content of This failure is now believed to be caused by a Itis believed that during the ultimate drying of the coating is increased.The viscosity may be increased merely by increasing the solids contentor it may be increased by the incorporation in the aqueous system ofmaterials which themselves have a relatively higher viscosity orwhichform an aqueous system of higher viscosity when dissolved in theaqueous phase of the coating. It has been found that the phenomenon ofthe tendency toward weakness at the plane 16 in the dried coatingisreduced if the initial layer of coating is high viscosity coating anda subsequent layer of lower viscosity coating is applied over the firstlayer while it is wet.

If it is attempted to apply relatively substantial quantities of thehigh viscosity coating to the sheet, the blade '12 must be applied tothe sheet with such low forces that the control of .the coating processis difiicult. In addition scratches and other defects which are commonlyexperienced during coating with smoothing or wiping devices mar thesurface of the coated paper. The application of only sufficient coatingin the first layer to provide the smoothing or leveling functionminimizes the depth or size of these'defects and permits substantialcoverage of such defects by the application :of a second layer of lowerviscosity cola-ting. Thus, one is able to apply the desired larger totalquantity of coating to the sheet without the disadvantages which arepresent in attempting to apply the same amount of coating eithercompletely as high viscosity coating with a blade coating device orcompletely with an air-knife coating apparatus and low viscositycoating.

As previously indicated, in accordance with the present invention, adouble-wet coating process is used to avoid the problems of the priorart noted above. In accordance with this process, a first layer of afluid or liquid coating composition of relatively high viscosity isapplied to the traveling paper. web (using, for example, a publicationgrade web) to be coated. The fluid coating of this first layer,(indicated after it is smoothed at 11a in FIGURE 3) may convenientlycomprise a liquid in which is suspended about 55 to solids by weight.Such a percentage of coating color produces a coating which preferablyweighs between 3 to 8 pounds per ream, i.e., 3 to 8 pounds of coatingper 3300 square feet of paper web. In actual practice, the weight of thecoating may range from a minimum of about 3 pounds per ream to apractical maximum of about 10 pounds per ream, above which thedifliculties hereinbefore referred to are encountered. This first layerof coating 11a may be applied conveniently and smoothed with aconventional blade coater (e.g. a trailing blade coater) positioned incontact with the side of the traveling web to be coated, while the otherside is supported firmly, for example, by a backup roll or similarsupporting means.

Although it is conventional in the paper making art to correlate solidscontent and viscosity and evenexpress the viscosity in terms of thesolids content, it is possible to introduce some variation in theviscosity of a coating composition of given solids content by theintroduction of materials into the aqueous system which will alter theviscosity. This may be done by conventional thickening agents, or it mayeven be done by variations in the actual quantity and/or physicalproperties of the adhesives that are dissolved or dispersed in theaqueous system. In the practice of the instant invention it has beenfound that the initial coating composition 11 that is applied by a bladecoater 12 should have a minimum viscosity of at least about 4000centipoises (as measured with a Brookfield viscometer at a rotationalspeed of 6 r.p.m., which is the basis for all of the viscosityfigures'hereinafter described). Coatings of this viscosity or more willbe found to be sutficiently viscous to avoid being influenced by theair-knife jet. The upper limit .of'the viscosities which may be used inthe initial coating composition 11 is a practical limit and, if desired,coatings of up to 40,000 centipoises viscosity may be used in theinitial blade coating step.

A specific example of the coating composition 11 used in the practice ofthe instant invention has the following formulation:

parts of clay 16 parts of soya protein 2.4 parts of 28% ammoniumhydroxide 1 part of calcium stearate 1 84 parts of water a avoiding themottled appearance which can result because of the application ofnon-uniform quantities of coating over a sheet surface to obtain arelatively smooth exposed coated surface. In the practice of theinvention, such a coating couldbe used as the initial coating 11 on akraft paper to obtain a smooth surface 15 of uniform color (which, inturn, could then be coated by a coating of any particular color desiredwithout the formation of a motside), which may be conveniently smoothedby the airknife so as to obtain a very high quality smooth surface 1%.The use of a higher solids coating in the first layer 11a followed by alower solids coating 19 in the second layer enables one to enjoy theadvantages of the air-knife coating process without suffering the poorerbonding of coating to fiber that results with low solids coatings. It ispossible, therefore, to reduce the amount of adhesive in the top coatinglayer without suffering reduction in the bonding strength. This has beenfound to be particularly advantageous in the production of coated papersof the invention which have improved receptivity to printing inks.

The coating composition 19 has a relatively low vis cosity and,therefore, relatively low solids content, such that if it were appliedin place of coating 11 as the initial layer the migration of adhesiveinto the web would result in a plane of weakness in the dried coatingfilm for the reasons heretofore described. In contrast, however, whenthis low viscosity coating 19 is applied to the relatively smoothsurface 15 of the still wet higher viscosity coating Ila, the coatinglayer lilo functions as a barrier to resist the rapid migration of theaqueous component from the composition I9 and into the web 1051.Instead, it is believed that the initial highly viscous coating 11yields a relatively smaller amount of its aqueous component to the webill-lilo which perhaps may be indicated by penetration only to the depthof the heavy-line 18a. In turn, the comparatively substantial aque uscomponent of the lower viscosity coating composition l? is not exposeddirectly to the capil ary or other adsorption action of the web becausethe initial coating layer Ila is interposed therebetwecn. The attractionfor water of the fibers of the web 1% is believed to be substantiallygreater than the ability of the particles in the coating layers Illa orto retain water, but the coating layers 11a and 19 contain particleshaving an approximately equal attraction for water, so that the aqueouscomponent of the layer 19 may be partially adsorbed or retained by theviscous layer llrz (if the water content thereof has been appreciablydepleted by the web llda), but that is approximately the extent to whichany migration of the queens component of the composition 19 takes place.Also, this rather limited type of water migration does not create anyweakness in the coating layer 19a closely adjacent the interface at thesmooth layer 15.

The second layer 19a of coating is applied to the coated sheet or webwithout any intermediate drying of the layer 11a and preferably at thesame coating station at which the first layer 11a was applied. Thissecond coating is preferably of about to by Weight in solid con tent andpreferably has a viscosity which is relatively low by comparison to theviscosity of the first layer 11a so that the second coating layer 19amay be readily handled in an air-knife coater.

A specific example of a second coating composition used in the practiceof the instant invention has the following formulation:

90 parts of clay 10 parts of titanium dioxide 4 parts of casein 12 partsof latex adhesive (Dow Latex 512L-solids basis) 06 part ammoniumhydroxide (28%) 2 parts ammonium stearate 156 parts water The abovecoating formulation has a solids content of about 43% and a viscosity ofapproximately 550 centipoises. The viscosity used for the second coating19 in the practice of the instant invention is limited practically tothe viscosity of coatings which can be readily handled by the air-knife2t). Such viscosities are within the range of about 100 to about 1500centipoises.

In the use of the specific devices shown in FIGURES l, 2 and 4 herein,it will be understood that the initial coating has a specificformulation for the coating 11 hereinbefore described and the subsequentcoating layer has the specific formulation for the coating 1) justdescribed. In addition, the initial coating Ill is applied in a Weightof approximately 5 pounds per ream, and the subsequent coating layer 1%is applied in a coating weight of approximately '7 pounds per ream. Inthe practice of the invention, the second coating layer 19a may beapplied in coating weights ranging from 3 to 14 pounds per ream.

In the practice of the instant invention the air-knife 26 of the typereferred to in the previously mentioned prior art patents is positionedto remove the excess of coating material 19 so as to obtain the desiredsmooth second layer 3%. The coating process is itself a doublewetcoating process, and it will be appreciated that the air-knife causes nopractical amount of drying itself in carrying out its function. Theair-knife 2t) merely re moves the excess over and beyond that desired.In addition, the air-knife 2th has a negligible effect upon the highersolid content of the first layer Ila. The first layer Illa is appliedand smoothed so as to obtain a relatively smooth surface 15. The coatingcomposition 19 is applied by means which will be described in detailhereinafter in such a manner so as not to effect substantial intermixingbetween the layers llla and 19a, and the air jet 20 smooths the layer Haas to obtain an extremely high degree of smoothness, again withoutsubstantiall' intermixing the layers flu and I911. Since the air jet 20acts primarily only On the second lower viscosity layer 19 which isspread over the already smoothed surface 15 of the first higherviscosity layer Illa, the leveling obtained in applying the firstcoating layer Ila permits the preparation of a much more leveled andhighly smoothed surface 21 on the layer 19a after the air-knife step. Asindicated, both coating layers may be applied at the same coatingstation, and even against a web traveling over the same backup roll. Inthe present process, the air jet 20 does not remove the coating materialfrom cavities or pores of the web base sheet surface as in the Case ofthe prior art because such irregularities in the web have been filled bya relatively high viscosity first coating layer 11a which has beensmoothed to provide a smooth surface 15 on which the relatively lowviscosity second coating layer 1941 may be applied in uniform thicknessby the air-knife directly, even while the first coating layer 11a isstill wet.

Several possible arrangements of apparatus suitable for carrying out thepresent invention may be used.

Referring to FIGURE 1, a web 3a} which is to be coated i trained arounda reversing roll 31 and a supporting roll or backup roll 32.. The firstcoating layer 11 may be applied by a duct or blade coater 33, the bladeof which is positioned closely adiacent the surface of the backup roll32 which firmly supports the back or uncoated side of the web The coater33 may be of the type disclosed in Trist US. Patent No. 2,368,176.

It should be appreciated that the trailing blade coater 33 is of thetype wherein a stationary element having a straight and. sharp trailingedge 33a acts in a wiping manner on the surface of the web 10. Coatersof this type have the capability of developing a maximum smoothing ofthe coating on the sheet. The blade coater of Trist is an excellentexample of this type of device.

The coating material may be supplied to the trailing blade coater 33 inany convenient or conventional man ner such as from a trough at thebottom of which the blade 33a is mounted. The web 30 may next be trainedover a coating roller or applicator roll 34 which is mounted to dip in atrough 35 containing the fluid coating suspension 19 to be applied asthe second Coating layer 19a. It will be noted that the same side of theweb 36 on which the first coating layer Illa was applied by the blade33:: is contacted by the surface of the roller 34 to apply the secondlayer 19 thereto. The coating or applicator roller 34 is a conventionaltype of coating device and may be of the type disclosed in US. PatentNo. 2,508,287, in greater detail. it will be noted that the applicatorroll 34 is mounted closely adjacent to the trailing blade coater V sideof the web 30 in a position to be acted upon by the.

air jet from the air-knife arrangement 37. The air-knife 37 (as well asthe air-knife previously shown diagrammatically) may be of theconventional type hereinbefore discussed and shown in detail in theprior art and will act to remove the excess of the second coatingcomposition 19 so as to obtain the desired smooth coating layer 19a.

Although the arrangement shown in FIGURE 1 is such that the second layerof coating 19 is applied to the web after it leaves the first supportingroll or backup roll 32, it will be seen from FIGURE 2 that both coatingsmay be applied and smoothed while the web is supported by a single roll.

Referring now to FIGURE 2, it will be noted that parts corresponding tothe parts shown in FIGURE '1 are indicated by the same reference numeralin the 40 series. It will be noted in FIGURE 2 that the web 4% istrained over a reversing roll 41 and then between the trailing bladecoater 43 and the supporting or backup roll .42, comparable to thearrangement shown in FIGURE 1. The applicator roll 44 which applies thesecond relatively low viscosity coating layer 19 is positioned in FIGURE2 immediately downstream from the trailing blade coater 43 and forms anip with the backup roll 12. The appli cator roll 44 is positioned inclose running relationship to the backup roll 42. It may be spaced fromthe backup roll beyond the thickness of the web or it may be run with avery low nip loading. This is to avoid compacting or compressing of theweb 40 at the second coating position to the extent that the firstcoating layer is disturbed. The roller coater 44 mounted submerged inthe coating bath in the trough 45 thus applies the coating composition19 as the second layer to the web 40 passing through the nip. Operationof the roll coater 44 without conventional roller coater nip loads isnecessary to avoid substantially intermixing the layers 11a and 19.Operation of the roller coater in this manner results in the applicationof relatively large quantities of coating to the sheet beyond suchquantities as would be applied by roll coaters used to produce coatedpapers of commerce. The details of the mounting of the roller coater 44are, however, brought out in previously mentioned US. Patent No.2,508,287.

It will, of course, be understood that all of the rolls herein describedmay be driven by any conventional means known to the art at suitablespeeds and that the pressure between the specific rolls 42 and 44 may becontrolled in any conventional manner known to the art.

Next, an lair-knife 47 is positioned in close running relation to thebackup roll 42 but again spaced slightly therefrom beyond the thicknessof the web and it is also.

positioned closely adjacent to the applicator roll 44. The air-knife 47applies a jet of air to the web 40 after it leaves the nip between therolls 42 and 44 and before it is separated from the surface of the roll42. Again, the air jet from the knife 47 functions to remove excessquantities of the second coating layer 19 applied to the web by theapplicator roll 44. If desired, the trough 45 may be ex tended under thepoint of contact of the air jet with the web to receive such excesscoating material.

Referring now to FIGURE 4, it will be seen that a somewhat differentarrangement is shown, In FIGURE 4 the reference numerals designatingparts corresponding to parts already described have the same number inthe 50 series. a For example, the continuously advancing web isdesignated by the reference numeral and it passes between a conventionalbackup roller 52 and a trailing 8 blade coater 53 wheie'at the initialcoating layer 1111 is applied and smoothed. Immediately adjacent to thisini tial coating device there is positioned an applicator roller 54submerged in the coating bath in a trough $5 so that it may apply thesecond coating 19 to the surface of the still wet initial coating layer11a. The coated web Sil then passes over a second backup roller 56.Again, an air-knife 57 is mounted in close proximity to the applicatorroller 54 and in close running relation to the backup roller 56 so as toremove excess of coating to obtain the ultimate desired thickness forthe second coating layer In FIGURE 4, however, there is added anadditional smoothing and spreading element in the form of a doctor blade53 that is positioned immediately ahead of the airknife 57 anddownstream from the applicator roll 54. The blade 5-8 serves as amechanical smoothing and spreading device for the second coating layer19 so as to remove a portion of the excess of this coating composition(such portion being designated at 1%). The doctor 53- and the coatingbath trough 55 are so mounted that this portion of the excess 1% mayflow back into the coating bath in the trough 55. The remainder of theeXCess is, however, removed by. the air-knife 57. This results in theextremely high quality smooth surface hereinbefore referred to inconnection withFiGURE 3 as 21. This remainder of the excess is indicateddiagrammatically at 19c in FIGURE 4 and itis shown being collected in asecondary trough 55a and returned through a suitable conduit'59 to themain trough 55.

7 It will be appreciated that the very light touch resulting in the highdegree of smoothness that is obtained using the air jet cannotordinarily be duplicated by the use of a blade, such as the blade 58.The operation of the blade is not ordinarily that delicate. On the otherhand, the blade 58 may be used to remove a substantial portion of theexcess 1% and thus simplify the final smoothing operation of theair-knife 57. The arrangement in FIGURE 4 permits the application of themaximum amount of coating in the second layer 19 by the applicator roll54. Initial smoothing of this maximum amount of coating may then becarried out by the blade 58 and final delicate smoothing may be carriedout by the air-knife 57. It will be appreciated, however, that the blade58 is operated with a delicate wiping touch so as to avoid substantialintermixing of the layers 11a and 19 at this position.

Nothwithstanding the misconceptions of the prior art, the instantinvention is predicated upon the discovery that the usual intermediatedrying step in plural coating operations is not required; Moreover,intermixing of the coating layers 11a and 19 which might have beenexpected on the basis of prior art knowledge does not occur to anysignificant degree in the practice of the instant invention. Thisis'primarily becausethe air-knife operation is controlled by virtue ofthe use of an initial highly viscous layer 11a that is not appreciablyeffected by the air-knife and the use of a second low viscosity layer 19which is readily effected by the air-knife By' simple adjustment of theair-knife, a very smooth level second coating 19a is achieved, which issuperior to that obtained by known single coating methods or pluralcoating methods involving intermediate drying steps.

In addition, the instant double-wet coating process using the finalair-knife coating step results in an overall lower drying load. Asindicated previously, the drying effect of the air-knife per se isnegligible; and drying is carried out subsequently to the air-knifestep. As an example of the advantagein this drying process, however, if6 pounds per ream of dry coating were applied to a sheet in the form ofa 60% solids composition and -an additional equal amount of coating wereadded as a second layer in the form of a-40% solids coating composition,the total water applied to the web would be only about 72% of thatadded, if a similar total amount of dry coating were added entirely inthe form of a 40% solids content composition.

As previously indicated herein, the applicator rollers 34, 44, 54 arerotatable. They may be rotated in either direction, but preferably theyare rotated in the direction of travel by any siutable drive means suchas the drive motor M indicated diagrammatically in FIGURE 4. Also, ithas been found preferable to drive the applicator rolls 34, 4-4, 54 (andparticularly the nip-defining roll 44) at a speed sufiicient to load theoncoming side of the coating roll contact with the web, i.e., to loadthe nip with the roll 44 so that a level of coating bath is maintainedat least slightly above the nip level at the oncoming side of the nip.This is not the normal bath level (unless a submerged nip is used) butit is a level (at L) in the immediate vicinity of the oncoming side ofthe nip that is maintained by driving the roll 44 at the necessaryspeed.

It should also be noted, although the instant invention is directedprimarily to the double-wet coating process, a single layer coater maybe used in the form of the embodiment of FIGURE 4 comprising only theapplicator roll 54 (which may be in nip-defining relationship with thebackup roll 56 in the manner of the rolls 44, 42 of FIG- URE 2, or whichmay be as shown in FIGURE 4), the blade 58, and air-knife or doctor 57all operating in conjunction with the backup roll 56. This single layercoater system could be used in the absence of the earlier trailing bladecoater 53 to apply only one coating to the web.

It will be understood that modifications and variations may be effectedwithout departing from the spirit and scope of the novel concepts of thepresent invention.

1 claim as my invention:

1. A method of double-wet coating one side of a flexible continuouslyadvancing web while firmly supporting the other side at a single coatingstation comprising, mechanically spreading on said one side of said weba first layer of a relatively high viscosity fluid coating comprisingabout 55% to 65% by weight of solids suspended in a liquid to provide asmooth surface on said one side of said web, applying to withoutsubstantially intermixing with said smoothly spread first layer while itis wet a second layer of a relatively low viscosity fluid coatingcomprising about 30% to 45% by weight of solids suspended in a liquid,and applying a jet of air to said coated web to remove only the excessof said second coating without substantially intermixing said coatingsand leave a residue of said second coating which is of uniform thicknessabove said first smooth coating and which consequently affords a smoothouter surface on said web irrespective of irregularities in the originalsurface of said web.

2. A method of double-wet coating one side of a web while firmlysupporting the other side comprising, mechanically spreading on said oneside of said web a first layer of a relatively high viscosity fiuidcoating composition to provide a smooth surface on said one side,applying to without substantially intermixing with said smoothly spreadfirst layer while it is wet a second layer of a relatively low viscosityfluid coating composition, and applying a jet of air to said coated webto remove only the excess of said second coating without substantiallyintermixing said coatings and leave a residue of said second coatingwhich is of uniform thickness above said first smooth coating and whichconsequently affords a smooth outer surface irrespective ofirregularities in the surface of said Web.

3. A method of double-wet coating one side of a flexible web while theweb is being firmly supported at and continuously advanced through asingle coating station comprising, smoothly spreading on said one sideof said web a first layer of a relatively high viscosity fluid coatingcomposition comprising a liquid and a suspended solid, thereafterapplying to without substantially intermixing with said smoothly spreadfirst layer a second layer of a relatively low viscosity fluid coatingcomposition comprising a liquid and a suspended solid, and applying ajet of air to said coated web to remove only the excess of said secondcoating without substantially intermixing said coatings and leave aresidue of said second coating which is of uniform thickness above saidcoating and Which consequently alfords a smooth outer surface on saidweb irrespective of irregularities in the original surface of said web.

4. A method of producing from a web having an irregular surface a coatedpaper having on at least one side a smooth surface coating layer ofuniform composition and of uniform depth comprising, spreading on saidone side of said web a first layer of a relatively high viscosity fluidcoating composition to provide a smooth surface on one side, applying towithout substantially intennixing with said smoothly spread first layerWhile it is wet a second layer of a relatively low viscosity fiuidcoating composition, and then applying an air jet to the second layerfor removing only the excess of said second coating layer Withoutdisturbing said first coating layer to leave a residue of said secondcoating which is of substantially uniform thickness above said firstsmooth coating and which consequently affords a smooth outer surfaceirrespective of irregularities in the surface of said Web.

5. A method of double-wet coating one side of a continuously advancingpaper Web, which comprises the steps of mechanically spreading on oneside of said web a first layer of a liquid coating having a minimumviscosity of 4000 centipoises and having a solids content of 55% to 65%to fill in surface irregularities on said Web and present a smooth outersurface for said first layer, next applying to without substantiallyintermixing with said smoothly spread first layer while it is wet asecond layer of liquid coating having a viscosity of to 1500 centipoisesand having a solids content of 30% to 45%, and apply a jet of air tosaid coated web to remove only excess of said second layer while it iswet and without substantially intermixing said layers and to leave aresidue of said second layer which is of substantially uniform thicknessand which presents a smooth outer surface irrespective of irregularitieson the original web surface.

6. A method of double-wet coating one side of a continuously advancingpaper web, which comprises the steps of mechanically spreading on oneside of said Web a first layer of a liquid coating having a minimumviscosity of 4000 centipoises to fill in surface irregularities on saidweb and present a smooth outer surface for said first layer, nextapplying to without substantially intermixing with said smoothly spreadfirst layer while it is wet a second layer of liquid coating having aviscosity of 100 to 1500 centipoises, and apply a jet of air to saidcoated web to remove only excess of said second layer while it is wetand without substantially intermixing said layers and to leave a residueof said second layer which is of substantially uniform thickness andwhich presents a smooth outer surface irrespective of irregularities onthe original web surface.

7. A method of double-wet coating one side of a continuously advancingpaper web, which comprises the steps of mechanically spreading on oneside of said web a first layer of a liquid coating having a minimumviscosity of 4000 centipoises to fill in surface irregularities on saidweb and present a smooth outer surface for said first layer, nextapplying to without substantially intermixing with said smoothly spreadfirst layer while it is wet a second layer of liquid coating having aviscosity of 100 to 1500 centipoises, and apply a jet of air to saidcoated Web to remove only excess of said second layer while it is wetand without substantially intermixing said layers and to leave a residueof said second layer which is of substantially uniform thickness andwhich presents a smooth outer surface irrespective of irregularities onthe original web surface, said steps being carried out while the otherside of said web is firmly supported by single rigid rotary surface.

8. A method of double-wet coating one side of a continuously advancingpaper web, which comprises the steps of mechanically spreading on oneside of said web a first layer of a liquid coating having a minimumviscosity of 4000 cenitpoises and having a solids content of 55% to 65%to fill in surface irregularities on said web and present a smooth outersurface for said first layer, next applying to without substantiallyintermixing with said smoothly spread first layer while it is Wet asecond layer of liquid coating having a viscosity of 100 to 1500centipoises and having a solids content of 30% to 45%, then mechanicallyremoving a portion of excess from said second layer withoutsubstantially intermixing said layers, and apply a jet of air to saidcoated Web to remove the remainder of excess of said second layer whileit is wet and without substantially intermixing said layers and to leavea residue of said second layer which is of substantially uniformthickness and which presents a smooth outer surface irrespective ofirregularities on the original web surface.

9. Apparatus for double-wet coating one side of a Web, comprising abackup roll engaging the other side of and firmly supporting acontinuously advancing web, mechanical spreading means in close runningrelation to said backuproll for applying a first layer. of relativelyhigh viscosity coating to the web to provide a smooth coated surface onone side of the web, second means positioned closely adjacent saidspreading means and in close running relation to said backup roll butspaced slightly therefrom beyond the thickness of the web for applying asecond layer. of relatively low viscosity coating to said first layerwhile it is wet without substantially intermixing the layers, andair-knife means in close running relation to said backup roll applying ajet of air to said second layer to remove only excess of said secondcoating with: out substantially intermixing said layers and to leavearesidue of said second layer which is of substantially uniform'thickness'and which presents a smooth outer surface irrespective ofirregularities on the original web surface. i V V i 10. Apparatus fordouble-wet coating one side of a web, comprising a backup roll engagingthe other side of and firmly supporting a continuously advancing web,mechanical spreading means in close running relation to said backup rollfor applying a first layer of relatively high viscosity coating to theweb to provide a smooth coated surface on one side of the web, anapplicator roll positioned closely adjacent said spreading means and inclose running relation to said backup roll but spaced slightly therefrombeyond the thickness of the web for applying a second layer ofrelatively low viscosity coating to said first layer while it is WetWithout substantiallyintermixing the layers, and air-knife means inclose running relation to said backup roll applying a jet of air to saidsecond layer to remove only excess of said second coating withoutsubstantially intermixing said layers and to leave a residue of saidsecond layer which is of substantially uniform thickness and whichpresents a smooth outer surface irrespective of irregularities on theoriginal web surface.

References Cited by the Examiner UNITED STATES PATENTS 1,942,383 1/34Dickhaut et al. 1l7102 2,279,553 4/42 Bradt 1l8 63 2,306,046 12/42Duggan et al. 11776' 2,425,231 8/47 Dickerman et al. 117l56 XR 2,679,2315/54 Pomper et al. 118-63 2,937,955 5/60 Loomer 117- 83 X 2,995,469 8/61Le Claire 11863 X 3,044,896 7/62 Warner 117-86 X RICHARD D. NEVIUS,Primary Examiner.

JOSEiH REBOLD, JOSEPH B. SPENCER,

. Examiners.

6. A METHOD OF DOUBLE-WET COATING ONE SIDE OF A CONTINUOUSLY ADVANCINGPAPER WEB, WHICH COMPRISES THE STEPS OF MECHANICALLY SPREADING ON ONESIDE OF SAID WEB A FIRST LAYER OF A LIQUID COATING HAVING A MINIMUMVISCOSITY OF 4000 CENTIPOLES TO FILL IN SURFACE IRREGULARITIES ON SAIDWEB AND PRESENT A SMOOTH OUTER SURFACE FOR SAID FIRST LAYER, NEXTAPPLYING TO WITHOUT SUBSTANTIALLY INTERMIXING WITH SAID SMOOTHLY SPREADFIRST LAYER WHILE IT IS WET A SECOND LAYER OF LIQUID COATING HAVING AVISCOSITY OF 100 TO 1500 CENTIPOLES, AND APPLY A JET OF AIR TO SAIDCOATED WEB TO REMOVE ONLY EXCESS OF SAID SECOND LAYER WHILE IT IS WETAND WITHOUT SUBSTANTIALLY INTERMIXING SAID LAYERS AND TO LEAVE A RESIDUEOF SAID SECOND LAYER WHICH IS OF SUBSTANTIALLY UNIFORM THICKNESS ANDWHICH PRESENTS A SMOOTH OUTER SURFACE IRRESPECTIVE OF IRREGULARITIES ONTHE ORIGINAL WEB SURFACE.